I am currently in the process of changing the air handler and heat pump in my house. I am working with a friend's father who does HVAC for a living. He came over and looked at my HVAC system and gave me a list of items to buy. I opted to go with a Goodman 3 ton split system. For the air handler I chose a Goodman ASPT - 3 Ton - Air Handler - Multi-Position - ECM Motor model #ASPT37C14. I saw that the dimensions of the air handler cabinet was 21 by 21. I also was told to get a UPFLOW FURNACE FILTER AND SUPPORT BOX - 21.5" X 28.5 for the return plenum which will sit underneath the air handler. I wanted to know if my return plenum was too big and he mistakenly had me buy one that was too big. Unfortunately my friend's father is on vacation and not answering and since he is putting my system in as soon as he gets back, I was wondering if I needed a smaller return plenum.
You do want your return larger this will not be a problem and allows room to install it then sheet metal or a flexible seal can be added. If things are two close it makes it tough to sweat the fittings